(1) Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof, and more particularly to an electronic device such as a power transistor having a high breakdown voltage, or a light emitting device such as a light emitting diode and a semiconductor laser device.
(2) Description of the Related Art
A power transistor having a high output and a high breakdown voltage has been widely used in high-output power supply circuits, automobile parts, and the like, and lower losses and higher breakdown voltage have been desired in the power transistor. Currently, a transistor which is commonly and widely used is a power metal-oxide-semiconductor (MOS) field-effect transistor using a silicon semiconductor. However, in order to further reduce on-resistance and realize a higher breakdown voltage, it is thought that a new material, which is typified by a wide-gap semiconductor such as a silicon carbide (SiC) or a gallium nitride (GaN), needs to be applied to the power transistor.
Among these new materials, the GaN-based compound semiconductor has a high saturated drift velocity, which makes it possible to realize a so-called high electron mobility transistor (HEMT) which generates two-dimensional electron gas in, for example, a heterojunction of aluminum gallium nitride (AlGaN) and GaN (hereinafter, referred to as a AlGaN/GaN heterojunction), so that the GaN-based compound semiconductor is expected as a material for realizing further reduced on-resistance. Research and development for a power transistor having a high output and a high breakdown voltage and including this GaN-based compound semiconductor are currently being conducted vigorously, which is disclosed in “Solid State Devices and Materials”, T. Murata et al., 2004, pp 261-262 (hereinafter, referred to as non-patent document 1), for example.
FIG. 1 is a cross-sectional view showing a structure of the conventional HEMT including the GaN-based compound semiconductor.
In this HEMT, on C-plane ((0001) plane) of a sapphire substrate 100, stacked sequentially are: an aluminium nitride (AlN) buffer layer 101; an undoped GaN channel layer 102; and an n-type doped AlGaN electron supplying layer 103. On the n-type doped AlGaN electron supplying layer 103, formed are: a gate electrode 104; a source electrode 105; and a drain electrode 106.
When, as the above-described HEMT, a device is manufactured using a GaN-based compound semiconductor, the GaN-based compound semiconductor is generally formed on a substrate having a plane direction of C-plane. However, if the GaN-based compound semiconductor is formed on the C-plane, a strong internal electric field is generated inside the GaN-based compound semiconductor, due to spontaneous polarization and piezoelectric polarization. As a result, in the above-described AlGaN/GaN HEMT for example, even in an undoped state, electric charge having a density of about 1013 cm−2 is generated in an AlGaN/GaN heterojunction. Therefore, the power transistor having a high output and a high breakdown voltage and including the GaN-based compound semiconductor, which has been researched, is a so-called normally-on field-effect transistor in which drain current flows when a gate voltage is 0 V.
Note that forming of a GaN-based compound semiconductor on a substrate having other plane directions except the C-plane is described in Japanese Patent Application Laid-Open No. 2001-160656, Japanese Patent Application Laid-Open No. 4-323880, Japanese Patent Application Laid-Open No. 10-275955, and Japanese Patent Application Laid-Open No. 7-297495, for example.